Known wind turbines are provided with a number of rotating systems. The blade pitch mechanism is one specific example of such rotating systems.
The rotor of a wind turbine includes a hub and a number of blades that are mounted on the hub. Although in some cases the rotor blades can be directly bolted to the rotor hub and then stalled thereto, the blades are usually attached to the hub through the pitch mechanism.
The pitch mechanism serves the purpose of adjusting the angle of attack of the blades according to the wind speed in order to control the hub rotational speed. This is carried out by rotating each blade around its longitudinal axis, that is, the axis extending from the blade root to the blade tip.
The rotational orientation of the blades for adjusting their angle of attack allows the load on the blades to be controlled. By controlling the blade pitch angle at any given value the hub rotational speed can be suitably controlled according to specific power production requirements.
Adjusting the angle of attack of the blades also serves the purpose of performing a rotor braking function. This is achieved by moving the blades through the wind turbine pitch mechanism into a blade feather position for protecting the blades from damages and wear which could lead to malfunction.
Known wind turbine pitch mechanisms typically include a pitch bearing. The pitch bearing is arranged between the rotor hub and the rotor blade for ensuring proper rotation of the rotor blade relative to the hub as stated above.
The pitch bearing generally includes a number of rows, such as two or three, of rolling elements, usually balls. The rolling elements transfer the torque from the blades to the hub and withstand the operating working loads. The pitch bearing further includes a number of bearing races, such as two: an outer, larger race bearing, and an inner, smaller bearing race. The rolling elements of the pitch bearing are provided between said bearing races.
In a pitch bearing of a common wind turbine pitch mechanism, one of the pitch bearing races, e.g. the outer bearing race, is connected to the hub, while the other pitch bearing race, e.g. the inner bearing race, is connected to a blade root portion (or sometimes to an extender).
As it is known, the angular displacement in blade pitching is small. The pitch mechanism of a wind turbine allows the blade to be rotated around its longitudinal axis from 0° to 90°. When the wind turbine is operating in normal conditions the blade pitch angle may range from about 0° to about 25° depending on the wind speed and therefore the power. When the wind speed is above 25-30 m/s the blade pitch angle may be 90° in order to stop the wind turbine rotor to protect the assembly. Therefore, not all of the rolling elements in common wind turbine pitch mechanisms are fully used.
When rotor blades are rotated through known pitch mechanisms high radial forces are generated. This results in high wear on the teeth of the drive pinion and the annular gear of the pitch mechanism. Loads are concentrated on specific areas of the bearing races which may lead to failure.
Bearings base their behaviour on very small contact regions, namely those of the rolling elements with the bearing races. Bigger bearings will be slenderer and therefore bigger deformations will be generated. This can undesirably compromise good load transmissions between rolling elements and bearing races.
In addition, machining of the balls is complex and materials used are expensive. Since blades are increasingly larger and heavier, larger bearings are required in terms of height and/or diameter. For example, in an offshore wind turbine, the diameter of the blades is of about 3 meters. Increasing the size of the bearings would result in an increase of the global size and therefore transport problems could arise. Increasing the size of the bearings would also result in greater deformations thus hampering transmission of the movement.
Bearings in common wind turbine pitch mechanisms comprising rolling elements are therefore not optimal.
As an alternative, plain bearings have been also proposed. They consist in providing first and second mutually rotatable elements, such as for example the hub and blade, or portions thereof, in a wind turbine pitch mechanism, with a layer of a hydraulic fluid film disposed therebetween such that the surfaces of said first and second elements slide against each other. Therefore, plain bearings do not make use of rolling elements.
U.S. Pat. No. 8,079,761 discloses the use of a plain bearing in a wind turbine rotor. The rotor bearing includes a main shaft capable of rotating relative to a bearing surface. A single fluid film is provided between the bearing surface and the main rotary shaft as well as a pump device. This allows the plain bearing to be selectively operated in hydrodynamic and hydrostatic modes.
However, the above solution is specifically intended for rotation of two elements in full turns and at high speeds. Working loads on such bearings are thus uniform on the inner bearing race. The variable action of the gravity has to be foreseen in this case for each angular position of the wind turbine rotor blades. The variable action of the wind speed which results in highly variable working loads must be also taken into account. The term variable as used herein relating to load vectors does not only refers to modules but also to directions.
The bearing disclosed in this document is therefore not suitable for small angle axial displacements in wind turbine applications. This bearing is not suitable for small displacements travelled at slow speeds combined with variable loads. This is because part of such bearing remains stationary so it does not change its position together with the substantially continuous movement of the bearing when in use. This solution does not provide the accurate high control of the rotating parts in a wind turbine when in use. This is very important since the blades are subjected to 3D movements derived from blade rotation and blade pitch combined movements.
There is therefore a need for a rotating system that can be used, for example, in a pitch blade mechanism, and/or other rotating parts in wind turbine applications, which can at least mitigate the above disclosed disadvantages.
The present rotating assembly is directed to rotating systems such as the pitch mechanism of a wind turbine which allows the above disadvantages to be at least mitigated.